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Mechanical Design for good RF Performance

Created; 09/02/2015, Changed; 05/07/2018, 16/07/2018
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Bakelite radio, Ultra MW/LW, built in loop aerial. Purchased in 1954 for £16, pictured working 55 years later when I last turned the radio on.

The heater current is supplied by a resistor and a negative temperature coefficient resistor (
NTC) in series with the mains supply. This is evident because the Ultra power light (that would be powered from the valve's heater power supply) initially comes on, then dims, then comes up bright just before the sound comes through, finally the light returns to a medium intensity. Small valve radio's like this usually have a live chassis (connected to live or neutral mains supply). This radio does not suffer with audio mains hum, that others like it invariable do, though the supply voltage mains ripple would be high. The sound reproduced is warm mellow, as if there were a large loudspeaker and substantial baffle (Valve sound, which I think, was developed by Philips in the 1930's). Inherent triode valve distortion was used to create sub-harmonic distortion that gives the impression that the radio has a better low frequency performance.

The circuit is identical to a 240V AC/DC model radio although the rating plate on this radio is specifies AC mains electricity supply only. A lot of these cheaper radios had a heater transformer but the most basic types like this one just uses series resistors to power the valve heaters. I have not turned the radio on since 2009 because is a capacitor could explode those old capacitors do not have a safety pressure relief device.

The Wireless Museum in Dulwich, West London is well worth a visit, I visited in the mid 1990's. They had working self seeking Radio, I had never seen one of those working before my visit. Working 405 line TVs, World War 1 valve manufacturing. The components are so fragile because they were cheap at the time but the museum manages to maintain these Radio's.

Electromagnetic Compatibility;

There are a number of strategies for minimising radio emissions and susceptibility to radio emissions. I intend discussing these points using various examples and small detours into other areas of electronics. It is necessary to separate circuit function, stability and electromagnetic comparability, though the symptoms can be similar. These things can interact with each other.
  • The Ultra radio has 0V bonded to the metal chassis everywhere. It is therefore filtered and screened very well within itself but then it is connected by a mains cord forming star point - no other way is possible or necessary. This is important to bear in mind and I discuss screening and filtering later.
Similarly to the radio with PCB design; Contain any susceptibility or emissions locally (to the PCB) by;
  • Use an embedded micro-controller so that all high frequencies parts are contained within the IC rather that use a bus memory and microprocessor solution
    • This also has the advantage in that, for the money you also, have a fully tested function block.
    • The highest frequency outside of the IC may be the crystal oscillator of say <5MHz rather than a bus of say >25MHz within the IC. 
    • Other high frequencies could come from output pins with some embedded micro-controllers and may require 50R series resistors in the output to mitigate this.
    • If ball grid array IC are used provide an 0V screen grid between the BGA pins, I have not designed anything using BGA and I do not know how well this measure mitigates given that the tracks are necessarily very thinner? See http://www.cherryclough.com/ - **** I have not found an on-line copy of the EMC UK's journals in which the article was written ****
  • Bandwidth limit the circuit using active and passive filtering to as much as is required. This is a cheap option so do it anyway!
  • The PCB should have at least an 0V plane.
  • Using PCB metal canning but do not just rely on the press fit contact but solder all around the lid (depending on the Can).
  • Screen enclosed switch-mode power supplies and other power circuits depending on the environment and the standards you are working to.  This may not be vital.
    • A 5mm clearance to the can or metal casing from an inductor matters a lot for a SMP <50W. The clearance of 5mm is particularly low grade metal casting.
    • Bond or screwing to the PCB at 10mm intervals maximum is recommended.
  • Use differential signals rather than single ended signals where practical. Even if it is not RF.
    • On PCB's run two wires together around the board. 
    • Terminate the ends with resistor output and capacitor input filtering. 
    • Particularly if you are unsure or inexperienced at PCB layout.
    • To make a point I make often get it right first time and then cut back later rather than get it wrong and fix.

Filter (bandwidth limiting)

At the boundaries, which are the input and output signals and connectors, use passive filtering, LC or RC, as opposed to active filtering, this is very important. Also keep the bandwidth limited to what is required throughout the circuit. Avoiding high Q filter circuits as they can ring at some frequencies (resonate).

  • Active Filter; is a filter that uses an operational amplifier to simulate an inductance or a high Q tuned circuit but using only capacitors and resistors. Active filters are generally designed to work over a limited frequency range and can behave badly outside that frequency range. Therefore active filters should have passive bandwidth limiting to keep the frequencies within the range that the circuit can accommodate to avoid transient intermodulation distortion for example; - A fast signal that passes though an operational amplifier stage without filtering but causing distortion because the op-amp could not respond to correct it and quickly enough

Transistorised Hi-Fi often use to suffer with susceptibility to lights and other thing being switched on or off producing radio interference and causing clicks to be heard on the Hi-Fi. Police and Taxi radio would "break through" on VHF transistor radio. The problem was that a bipolar junction transistor could rectify radio interference and thereby act as a detector (diode). A fix that Electronics technicians, in the 1970's used with amplifiers, was to fit a 100nF ceramic capacitor between the Base and Emitter pins of the first transistor. I have also seen 100pF capacitor fitted similarly built into a design for an Eagle amplifier.

Positioning of components is also important.


 Small signals --- Switch mode driver IC --- Switching power and Inductor --- Input and output capacitors --- Filter section --- Input/Output connections. 
  • The mounting holes - More could be fitted to improve EMC (a mounting hole every 10mms is recommended) but often what should be ideal is not practical and may not seem to work. To avoid instability in the control loop thereby contributing to EMC issues the two mounting holes on the left may need to be isolated from metal work. But if such a compromise is made there is a risk that another EMC issue will arise - this is the problem with reverting to such a star point strategy but bonding every 10mm is still much less risky than starting with a star-point 0V strategy.
  • Signal and Power switch --- Low current and switching drive are kept apart.
  • Circulating power around the Inductor --- PCB tracks need to be short thick and straight.
  • Filter and energy reservoir --- For loop stability and the main part of the containing and smoothing out internal switching pulses.
  • Segregation of internal RFI and external RF ---  I/O noise does not cross. Internal noise does not cross.
  • Optimal ground point if multiple 0V connections don't work --- Is near the input and output. What happens in practice can depend on apertures in nearby metal work and the qualities of the metal used in the casing.
The above PCB is for a switch mode power supply for LED driving. The design has a number of features to achieve low electromagnetic emissions. (AL-0022-01B) created using CADSTAR 16 express. The design is most certainly over filtered and is the best way to design because it is easy to reduce to the size and costs on filtering rather than to add it later with a good PCB layout that was not designed to have those additional parts. But don't over design in things that you probably won't need later.

Spot the omission - the circuit can have PWM driven to control the LED intensity. The control both isolates the LED using a transistor and also turns the regulator off during the off period. The usual mechanical dilemma applies and there is no space to do what is desired and there is little or no over voltage protection. The solution is you have got to talk to the mechanical designers and stylists (the later people can stitch you up) both press your point and offer work-around's and discuss the problems with those or express the compromises. 

This sort of function can be placed on a PCB with other RF and signal circuits and although the power planes should not be split I've seen it done and the engineers who did that were not happy but it was expedient. In this case the input output and power would be connected on the left but an isolation to the planes put around the other three sides. But if you can put the power supply block on corner edge and away from sensitive circuit that is better. Put very sensitive circuit on cable and coupled through common mode choke then bonded to a metal case is very good.

EMC and compromise might be expedient - this works but is not recommended.  This sort of function can be placed on a PCB with other RF and signal circuits and although the power planes should not be split I've seen it done and the engineers who did that were not happy but it was expedient. In this case the input output and power would be connected on the left but an isolation to the planes put around the other three sides.

If you can put the power supply block on corner edge and away from sensitive circuit that is better. Put very 
sensitive circuit on ribbon cable to a PCB and whatever sensor you are using mounted on that PCB is very good. Connecting the ribbon through common mode choke then bonded to a metal case is very good. But also a star point but no common mode choke is, once again not recommended and is also very good.

I can think of an example where if a motor and encoder were moved to where the microprocessor was
 at the other end of an instrument away from the detector that in every respect the instrument should 
worked better. But there was a styling constraint that prevented that happening originally. It was already 
an excellent instrument and it had been improved over time and always was much more sell-able than 
had been anticipated. Later a desire was expressed to develop another but higher performing instrument but
 such was not going to be possible without the addition of a mu-metal screen the re-arrangement of some electronic 
and software but the design time was not available. In the end those things were understood, I made sure of that.

If necessary a capacitor may be placed between the 0V plane and the connection to chassis to provide DC isolation (ensure that this part meets any standard for the industry and application). But using the mounting points as shown this could increase emissions a solution is to fit a zero ohm link that can be removed but firstly add more mounting points and work on other things before resorting to isolating any mounting points. RF emissions in the Near field are not predictable and a metal enclosure for a power supply can make em worse. Firstly though ensure that the controller control loop is stable that is optimally compensated or a little over damped. If PWM for LED brightness control ensure that the control loop does not RAMP but switches between on and off (this also ensure that the LED colour is stable).
  1. The input and output are close together so that the DC current path is short and the AC path is very short (on the right hand side).

  2. The tracks that carry current and particularly switching currents are short fat and straight (Left side next to the IC). These track widths can be varied over there length the EMC performance might be improved by so doing. But the effect of varying the track width is likely to be marginal and in practice it is not done at these modest frequencies.

  3. This circuit is used to drive LED's. In boost mode the input current flows almost continually and the circuit is designed to have minimal switching frequency ripple. In Fly-Back mode the AC current is also arranged to flow almost continually although the DC current is switched. This Diodes Ltd. design should achieve a better performance than a SEPIC design although their may be an increased current ripple on the LED passed through from the input supply.

  4. The transistor on the right provides PWM drive of the LED but avoids current ramping that would change the colour of the LED. (The LED colour changes with current so dimming is achieved by pulse width modulating the LED current)

  5. See Diodes Ltd. data sheet for; ZXLD1374 http://diodes.com/datasheets/ZXLD1374.pdf The first diagram shows the AC path with the output capacitor connected from Vout+ to 0V but the DC path is Vout to Vin+. The PCB above also has protection against reverse supply with an input diode, this diode also prevents supply dips from causing over current pulses in the output LED.

  6. Figure 2 http://diodes.com/_files/evbd/ZXLD1374EV User Guide Issue 3.pdf The circuit and component numbers are different on my PCB above which has an additional inductor between D1 and C5 and their is a transistor in the final output for PWM drive of the LEDs and another filter network in order to meet Automotive standards for emissions.

  7. There are a lot of vias from 0V on the top and underneath. This is to make the 0V plane UN-fragmented and to provide as much copper in the 0V as possible.

  8. Place associated components together. Keep path lengths short. The main point here is that you have to prioritise and choose were you can make compromise. Therefore spend time on the PCB layout to minimise the track lengths, bends, and use thicker track where there are fast voltage and current transitions. Insure that the 0V return current can follow the same path as the signal current by using unbroken ground planes ( the current will then find its own way back and at high frequency stick close to the signal track path). That way you can minimise the amount of compromises. Also using differential pairs for signals like transmission lines is an excellent approach in addition.

  9. Some capacitors and made up of three components two large value capacitors plus one small NPO type to maximise filtering. This is probably unnecessary one large and one small value should be fine and maybe just one large value capacitor will also be fine. The switching power diodes are Schottky types these have better switching characteristic that does not look like ringing compared to other very fast types of diode.

  10. PCB design - make a ground plane and spend time minimising and fragmentation of that plane. If you need to add tracks in a ground plane make them short and bring them back to routing layer(s) as soon as possible. Therefore minimise or avoid make holes in the plane layer. Actually the best strategy is to have two inner power plane layers, and two outer routing layers. Putting the planes on the outside, apparently can cause buckling when soldering in an oven, but also components make holes in the planes which should be avoided as this would be fragmentation the plane. A top track layer and a bottom 0V plane layer works fine.

  11. One tip, if you can probe your circuit with an oscilloscope with the ground connection to anywhere on the 0V plane and there is no difference in the waveform observed then you have understood and got a good PCB layout. You should also find that pulse waveforms don't ring (overshoot many times), but some overshoot may be acceptable. Think about this before you manufacture your PCB. I shall discussing this later.

  12. Radio's like the Ultra above is physically laid out so that the RF input is followed by the mixer, IF amplifier, detector, Audio amplifier, to the output valve and finally the power input rectification smoothing and filtering. So the power supply is near where the signal is largest at the output, and furtherest away from the RF input where the signal is smallest and most susceptible to interference.

Home made Oscilloscope based on a Practical Wireless design of about 1973.

I worked on the design over a number of years, improved it greatly. By 1977 it was evident that I had learnt a lot but this was never going to be a particularly good Oscilloscope primarily because of the low intensity of the CRT, and I stopped working on it. The bandwidth is increased from less than 10KHz to over 500KHz, High voltage CRT supply was changed to a high frequency type, low noise, sine wave, with proportional control, to eliminate Z-modulated mains-hum, and the voltage increased. Input transformers relocated to under the chassis back left as far away from the CRT as possible to eliminated X and Y axis modulation. Improved trigger and time base. Grounded everywhere (approximately because I was quite in-experienced), and all modification made with recycled parts. The next improvement would have been to fit a larger transformer, which would have had to be nearer the CRT, and I had some Mu metal to screen that, as everything was under stress.

Grounding strategy - Using one point (star ground) or grounding everywhere.

A star point, connection to a single point on a chassis tends to give very good low noise performance but there will be resonant frequencies and this is very difficult to deal with. Alternatively grounding everywhere can give a higher noise level, because there may be varying current across chassis that inject low level voltage into the circuit, but resonances are likely to be a lot less this configuration is currently more in favour. I recommend that you pick and choose but generally go with bonding everywhere.

 

Stereo Amplifier with tone controls, balance and volume pre-amplifier.

About 5+5W into 4-16 ohms I believe?

Mullard amplifier - these modules or reference designs are classics, they used minimal number of components, no 0V plane on the PCBs but good bandwidth limiting and star point 0V strategy. I continued with that star point strategy in connecting the modules having tried grounding everywhere unsuccessfully so under guidance of my manager at my place of work, I would have been about 18. The amplifier works very well. One of the power amplifier's is not fitted and there are some disconnected wires.

The output stage is likely to be a push-pull NPN/PNP transistor output stage probably with the output transistors base bias provided by the transistor base bias connected to the loudspeaker forming bootstrap biasing. There is a snubber network across the loudspeaker in this case it is called a Zobe network and ensures that the amplifier works in a predictable way despite the inductance of the loudspeaker (the resistor's value is equal to the loudspeaker's resistance and the capacitor is 100nF presumably balancing the speaker inductance). The pre-amplifier may be a module LP11 cut in half and stacked for space constraints.

The mains transformer is a spilt bobbin type designed that way to minimise electrostatic noise coupling between the primary and secondary windings. I was very unaware of safety with mains as is evident but is typical of hobby electronics. For example earth bond should not support anything but just carry the earth conductor and use crimped connections bolted with anti-shake washer connections - or just use an external mains power supply. There are plenty of used power supplies and other components that can be re-used.

For example Radio's like the Ultra above, have a steel chassis which is signal ground (0V). When ever 0V (possibly connected to mains live or neutral) is required there is a short connection to a nearby point to chassis. My home made oscilloscope is also bonded every where (approximately - I was inexperience). Therefore neither design use much star point for grounding and both work well. A strategy of using differential signals when bonding everywhere, like you would have to with Star point anyway, also improves the outcome. You can place common mode chokes between PCBs and provided any common mode current due to voltage differentials across the metal work that would flow within the common mode chokes is less than the rated saturated current there will be benefit.

Pictured below is an RF PCB layout for a very low power crystal oscillator and Phase Locked Loop of a microcontroller. The PCB layout is an improvement on another Motorola (Freescale) example that used star point and made a point of having no 0V plane under the crystal oscillator section of the board. But it still is not an optimal adherence to the rule of RF design should be bonded to 0V everywhere though it no doubt works, but I don't recommend it. This design has complete the 0V guard track to form a ring around the crystal but only has one connection to the 0V plane - I don't see any merit in laying out the oscillator this way but I have used an earlier similar reference design successfully. Do ensure that there is 0V plane as the next layer below the routing plane shown and that it is not fragmented and don't place any other tracks under the crystal so as to prevent 0V return currents flowing under the crystal - the main point made in most applications notes.

http://www.freescale.com/files/microcontrollers/doc/app_note/AN2727.pdf (Page 21)

This application also suggests star point for the analogue systems but it works fine with the 0V supplies taken by the shortest path to 0V plane and the supply decoupled and connected to the supply pin via a small series resistor, instead. Not only is this approach I suggest correct but it works. On some occasions though you do have to follow the application note - particularly when there is a power supply start up order and the RC network on the analogue supply makes the order of start up wrong Or that it simply won't work as well any other way. The latter case comes about because (my theory) star point produces very low noise frequency bands and tuned notches as I say elsewhere, provided a tuned notch is avoided this may be the only way to achieve the performance required of some Delta Sigma or Sigma Delta A/D converter types.

A tip given to me for when it is necessary to use star point in an RF environment and that is add a bead choke - this is low Q constructed choke using a soft magnetic material such as used in mains filters. Alternatively add series resistors and keep the signal paths short and like a transmission line grouped together, following the same path. Also common mode chokes intended for filtering are similarly low Q and can be placed at the output or the input of board to board ribbon or multi wire cables and in that way each board can be connected to chassis.

Examples of enclosures and magnetics;

Grounding every where (connecting 0V to metal work and bonding everywhere not specifically earthing) is best but grounding at one point has been common practice since 1960 until the turn of the century. Power Inductor can induce current into enclosure, so keep them some distance from metal enclosures. It will be found that grounding at many points does not work in a small enclosures and the work around was to ground at one point were the power a signals came in - So I use some star point if really necessary but not as a starting point.

Keep some distance between wound components to minimise the coupling between them. Some power input soft common mode chokes are wound with the former in a horizontal plane whereas power chokes are wound with the former in a vertical plain in order to somewhat null and thereby minimise the coupling between them. It may be possible to have three wound components in the three possible planes but it is better to put distance between those components you can use both approaches. The PCB layout for the LED PSU above shows wound components that are wound in the same plain, except for input bead chokes. In that design it is not possible to source ideal wound components but a design like this still can achieve very low radio emissions. In the PCB design I have phased the input and output chokes to behave as a common mode choke if there is coupling between them but I have not experimented to see if there is a measurable benefit in doing that?

Useful Links;

  • http://www.cherryclough.com/  I came across this company at Electronics Weekly sponsored show in June 2009. They have an excellent understanding of EMC issues and solutions.


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